Q9 of 28 Page 159

What is potential energy? Derive an equation for gravitational potential energy of a body of mass ‘m’ at a height ‘h’.

Potential energy of a body is its ability to do work, whereas gravitational potential energy of an body is its ability to do work in gravitational force.

Consider a object of mass ‘m’ raised to a height ‘h’ above the ground. The minimum force required to raise the object is mg. The object gains energy equal to work done on it.



W = Force Displacement


W =mgh


W = mgh


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7

One person says that potential energy of a particular book kept in an almirah is 20 J and other says it is 30J . Is one of them necessarily wrong? Give reasons.

 8

In which of the following cases is the work done positive or zero or negative?

a) Work done by the porter on a suitcase in lifting it from the platform on to his head.


b) Work done by the force of gravity on suitcase as the suitcase falls from porter’s head.


c) Work done by the porter standing on platform with suitcase on his head.


d) Work done by force of gravity on a ball thrown up vertically up into the sky.


e) Work done by force applied by hands of a man swimming in a pond.

 10

When an apple falls from a tree what happens to its gravitational potential energy just as it reaches the ground? After it strikes the ground?

 11

Let us assume that you have lifted a suitcase and kept it on a table. On which of the following does the work done by you depend or not depend? Why?

a) The path taken by the suitcase.


b) The time taken by you in doing so.


c) The weight of the suitcase.


d) Your weight.